Carbon nanotubes have received significant attention for technological applications because of their desirable properties which include high electrical conductivity, high carrier mobility, high mechanical strength, and ability to be processed into various forms such as fibers and thin films. Carbon nanotubes in the form of networks and films have been proposed as the electrodes for several types of devices including polymeric supercapacitors, and as transparent electrodes for organic light emitting diodes, organic photovoltaic devices and organic electrochromic devices. Also, carbon nanotube dispersions within an electroactive organic matrix, such as poly(alkylthiophene)s and poly(phenylene vinylene)s, have demonstrated potential as an electroactive component within bulk heterojunction photovoltaic devices. Recent work has also demonstrated that dispersing carbon nanotubes within an organic polymeric matrix (such as polystyrene and polyacrylates) dramatically increases, among other properties, the strength, toughness, and durability of the organic polymer. Therefore, it is anticipated that the dispersion of carbon nanotubes into electroactive organic materials to produce materials which are active in charge storing supercapacitors/batteries, solar cells, electrochromic fiber and film-based devices, and light emitting devices, aside from producing enhanced electronic properties, would result in durable and robust materials.
In such devices it is necessary to couple an organic material to the carbon nanotubes electrically. Such electrical coupling requires intimate proximity between the organic material and the nanotube surface. The nanotube surface, like the basal plane of graphite, is a low energy surface that interacts only weakly with many of the known organic materials that are most useful in such applications. This weak interaction can result in poor contact, also known as poor wetting, between the organic material and the nanotubes. For example during a deposition of the organic material onto the surface of a nanotube network electrode, the organic material can bead up along the nanotubes leaving sections of nanotube that are unevenly coated with pinholes or larger sections of nanotubes that are not covered by the organic material. Such uncovered sites can be detrimental to the device performance because, among other disadvantages, pinholes can result in electrical shorts between the nanotube and the counter electrode. Recent work by Zhang et al. Nano Lett. 2006, 6, 1880-1886 and Li et al. Nano Lett. 2006, 6, 2472-2477 has demonstrated that in photovoltaic devices and light emitting devices, hole transport layers such as those based on PEDOT:PSS, when deposited as thin films onto carbon nanotube network electrodes, can reduce the occurrence of pinholes by planarizing the electrode surface. This deposition process essentially covers the nanotubes completely with a thick even layer of polymer. Such a deposition is a common practice for devices constructed with high surface roughness ITO/glass. However, the device performance reported was inferior to that of the ITO analogs, and well below the performance required to make such devices commercially viable. Although the deposition reduces the pinhole problem, the use of polymers such as PEDOT:PSS on carbon nanotube films displays some disadvantages including:                a) The very high effective surface area of nanotube network electrodes for charge injection is significantly reduced by planarizing the nanotube surface with PEDOT:PSS.        b) Interfacial adhesion of the PEDOT:PSS with the carbon nanotube surface is poor, possibly due to the strongly polar and hydrophilic nature of the PEDOT:PSS.        c) Mechanical deformations, such as shear stress and bending can induce delamination of the PEDOT:PSS from the nanotube substrate, due to the lack of affinity between the nanotube surface and the PEDOT:PSS resulting in damage or destruction of the device.        d) The strongly acidic poly(styrenesulfonic acid) matrix of PEDOT:PSS can promote device decomposition.        
An alternative approach to PEDOT:PSS deposition for resolution of the pinhole problem is the coating of the nanotubes with a thin layer of parylene as disclosed by Aguirre et al. Appl. Phys. Lett. 2006, 88, 183104 in an electroluminescent device. Although parylene provides a coating that improves the coupling of the organic layer to the nanotube surface, it is an insulator and blocks electron and hole transport across the nanotube/organic layer. Devices containing dielectric polymer layers require higher voltages to permit current flow through the insulating layer increasing the device turn-on and operating voltages. This higher voltage increases the likelihood of device decomposition through Joule heating or other pathways and can also require high power for operation. The higher power and heating are two characteristics that are undesirable for electroluminescent devices such as displays.
Recent efforts have addressed the poor interface between organic materials and carbon nanotubes in a number of ways. For example, covalent functionalization of the carbon nanotube surface has been shown to improve the dispersion of poly(3-octylthiophene) and C60 in bulk heterojunction solar cells by covalently modifying the carbon nanotube side-wall. Unfortunately, device performance was poor, as chemical modification of the carbon nanotube side-wall introduces conjugation disrupting defects that decreases their conductivity.
An alternative method for improving the interface between organic molecules and carbon nanotubes has been through non-covalent functionalization with pi interacting organic molecules. Substituted polycyclic aromatic hydrocarbons, generally being pyrene or related derivatives, have been shown to provide non-covalent interaction with the nanotubes to permit association of other molecules with the nanotubes while minimally impacting intrinsic electronic transport properties. Such non-covalent functionalization of carbon nanotubes in photovoltaic devices has been explored by employing monomeric pyrene derivatives that are cationic quaternary ammonium salts adsorbed to the surface of carbon nanotubes, followed by a layer-by-layer deposition of an anionic polythiophene derivative to form a composite material. The resulting composite photovoltaic device exhibited modest performance.
Applications that rely on small-molecule pyrene derivatives associating with nanotube surfaces are limited by the association/dissociation kinetics of pyrene from the nanotube surface. Essentially, when a monomeric pyrene moiety dissociates from the nanotube surface, it can diffuse away from the nanotube and can be essentially lost to the system. Inevitably, monomeric pyrene derivates can dissociate from a nanotube surface over time, especially in solution or in the case of a high electric field device where an ionic pyrene derivative can migrate towards an oppositely charged electrode, destabilizing the interface between an electroactive polymer and a nanotube. In contrast to monomeric pyrene derivatives, an oligomeric or polymeric derivative with multiple pyrene moieties per polymer associated with the nanotube surface could possess many orders of magnitude higher association constants to that of a monomeric moiety. This enhanced association of a polymeric moiety has been demonstrated in a few examples using non-conjugated polymeric systems such as poly(methyl methacrylate) and polystyrene as disclosed in Lou et al. Chem. Mater. 2004, 16, 4005-4011. Wang et al. J. Am. Chem. Soc. 2006, 128, 6556-6557. These systems focused on enhancing the dispersion of nanotubes into solvents and demonstrate that polymeric derivatives containing multiple pyrene derivatives exhibit extremely stable non-covalent interactions with carbon nanotubes. This affinity is so strong that, in one example, the nanotube/polymer material was reported to have had to be heated to over 250° C. to essentially “burn off” the polymer.
A need remains for a system where electroactive, conjugated, or conducting polymers (CPs) have high association constants with carbon nanotubes, and a process to provide the same. Such a material would be useful as CP/nanotube composite materials for electroactive and related devices including: electroluminescent devices; photovoltaics; electrochromic films and fibers; field-effect transistors; batteries; capacitors; and supercapacitors.